1. Field of the Invention
The present invention relates to a method for preparing a profile in communication based on wide-band code division multiple access (W-CDMA), specifically to a method for preparing a profile enabling the signals from a handheld phone (mobile unit) to synchronize with the signals of a base station.
2. Description of the Prior Art
Generally, in communication based on W-CDMA, the signals from mobile units and the signals from base stations are not in synchrony with each other. Therefore, if a mobile unit (mobile station) seeks a connection with a most proper base station, the unit must identify the base station and try to synchronize its signals with those of the base station. According to W-CDMA, if the mobile unit wants to establish communication, and identifies the most proper base station and tries to synchronize its signals with those of the base station, the unit prepares a profile to support the synchronization.
FIG. 10 shows a conventional format of such a profile comprising frames, slots and symbols to be exchanged between the mobile unit and the base station. As shown in the figure, according to this data format, one frame has a length of 10 ms, and two frames or a transmission time interval (TTI) have a length of 20 ms (the number of frames totals 4096). The frame (e.g., frame #0) consists of 15 slots, and a slot consists of 10 symbols. Incidentally, a slot has a length of 667 μs and consists of 2560 chips, while a symbol has a length of 67 μs and consists of 256 chips.
When the mobile unit synchronizes its signals with those of the base station, it uses a primary-synchronization channel (P-SCH) formatted as described above and transmitted by the base station. The P-SCH is a physical channel which supports the signals of the mobile station to be synchronized with those of the base station. The P-SCH formatted in the same manner and having the same primary synchronization code (PSC) is spread commonly by all the base stations. The PSC is a 256 chip code, and the same for all the base stations, and for all the synchronization channels (SCHs) of the slots.
Actually, only the leading symbol (256 chip) of each slot is transmitted. The above described P-SCH consists of I- and Q-channels intersecting each other at right angles both of which spread the same code and transmit the same data.
FIG. 11 shows the relation of norm values with timing values upon which the mobile station synchronizes its signals with those of the base station using the data format as shown in FIG. 10. As shown in the figure, if a mobile station finds a communicable base station, it resorts to reverse diffusion for a length corresponding to one slot interval (10 symbols×256 chips). Only when reverse diffusion is carried out at a good chip timing enabling the synchronization of two signals here concerned, a high correlation is observed between the values detected through the I- and Q-channels, and the mobile station can successfully synchronize its signals with those of the base station.
Since the base station transmits the same signal through the I- and Q-channels at the same timing, it is possible for the mobile station to detect the synchronization timing with a high precision by receiving the I- and Q-channel signals separately, determining their correlation, and taking the result as a norm value (which may be plotted as a vector on an I-Q plane). This synchronization step is called Step 1 according to the 3rd Generation Partnership Projects (3GPP).
FIG. 11 represents the norm values (ordinate) as a function of timing values (abscissa): the total length of timing values is equal to one slot interval, and the norm value exhibits a peak corresponding to a base station (BTS A).
Generally, since the over sampling ratio (OSR) or the smallest sampling interval with respect to a chip length is over one unit, the total sample number for the one slot interval will be equal to 10 symbols×256 chips×(OSR). Further, because it is often impossible to determine the most proper base station by simply following norm values for a period equal to one slot length, it is necessary to improve the detection sensitivity by tracing norm values for several slot lengths, and accumulating the results for those slot lengths.
For example, FIG. 12 shows the addition of two norm values each representing norm values obtained for a different slot length. As shown in the figure, it is possible to cancel out noise components by adding the norm values obtained for two different slot lengths, which will enhance the sensitivity of detecting a possible peak.
FIG. 13 is a block-diagram of the principal parts of a conventional mobile unit responsible for the preparation of a profile with which the mobile unit can synchronize its signals with those of the base station. FIG. 14 is a sketchy flowchart representing the steps taken by the conventional mobile unit as shown in FIG. 13 for preparing a profile which is required for synchronization. As shown in FIGS. 13 and 14, the conventional circuit responsible for the preparation of a profile includes a profile data preparing portion 1a which prepares a profile based on timer values and new norm values fed as input. To the profile data preparing portion 1a are connected a profile memory 2a for providing a previous cumulative value and storing a current cumulative value, and a peak detection portion 3a which receives a current cumulative value from the profile data preparing portion 1a for storage, and delivers timing and norm values to the internal circuit of the mobile unit such as a handheld phone. When the unit compares a current cumulative value with a value retained in a peak timing and peak norm register 10, and finds that the current cumulative value is larger than the latter, the unit replaces the value retained in the register 10 with the current cumulative value. On the contrary, if it finds the current cumulative value is smaller, it will allow the previous value to stay in the register as before.
The profile data preparing portion 1a includes an N slot detector 4a (N represents the number of slots by which norm values are added cumulatively) equipped with a write start timer value register 5 which delivers, in response to timer input, a write enable signal (low active) WRB and a cumulative addition start timing value SST as output, and an adder 6 which achieves cumulative addition by adding a new norm value to a previous cumulative value fetched from the memory 2a. The peak detection portion 3a includes two peak registers 10 comprising a peak timing register and peak norm register, and retains a current cumulative value together with its timing value, and delivers, in response to an end flag (which may be replaced with a WRB), a prepared timing value and norm value as output to the internal circuit of the mobile unit.
In short, a new norm value fed as input is accumulated on a previous value at the same timing within each slot interval, and the accumulation result is written into the profile memory 2a. Incidentally, for the case shown in FIG. 13, OSR is 4, and the profile memory has a word width sufficiently large to contain 10240 samples.
Data flows as shown in FIG. 14: a new norm value is fed as input (step S20); a previous cumulative value is fetched from the memory 2a (step S21); and the new norm value is added to the previous cumulative value (step S22). A current cumulative value is obtained as a result of the procedure (cumulative addition), and this value is stored in the memory 2a as a profile input (step S23).
For this operation, however, the bit width of the profile memory 2a which can be expressed by the above equation is necessary. For example, if a newly fed data (new norm value) has a bit width of 16 bits, and the maximum passable slot number N is 64, the bit width of the profile memory should be 22 bits. Thus, in total, the profile memory should have a capacity to accommodate 10240 words×22 bits.
As long as the profile preparing circuit prepares a profile based on norm values thus obtained, the profile memory is urged to have a big capacity which poses a problem. Since the mobile unit mainly consists of handheld mobile phones, it is desirable to reduce the demands for its internal circuits to a level as low as possible.
However, to meet such a demand, if floating point representation is introduced so as to round off numerical data, the peak detection sensitivity will be impaired. Therefore, there is a demand for a new method which does not depend on rounding-off but allows the cumulative addition of norm values without imposing a heavy burden on the internal circuit elements.
If it is required to cumulatively add new norm values without modifying them, and cumulative addition is repeated N times, or for the number N of slots (N: the number of slots for cumulative calculation), the required bit width of the profile memory will be as described in the above equation 1. In that equation, a new norm value is fed into the input data bit width.
A new norm value is not likely to have a maximum value; and it is also unlikely that the peak of a new norm value corresponds in timing with the peak of a previous cumulative value. Therefore, out of the samples allocated to the profile memory, those allocated to higher bits often remain zero, or conversely those higher bits remain unoccupied all through the cumulative addition operation.